CWI Cryptology Group

Abstract: We will introduce quantum computers and quantum algorithms from a theoretical perspective. First we explain how computational concepts like bits and operations can be realized in the context of quantum theory, and then we sketch the main quantum algorithms that we have today: Shor's algorithm for factoring, Grover's algorithm for search, and some subsequent algorithms. The existence of these algorithms is one of the main motivations to actually try to build quantum computers in the lab.

Abstract: Fifteen years after the discovery of Shor's algorithm, experimental physicists have implemented various quantum protocols in the lab, including quantum factoring. In this talk, I will discuss what "knobs" we have available in the lab, and how we can use these knobs to implement quantum protocols. I will also briefly summarize the state-of-the-art in various physical realizations of quantum bits, and comment on future prospects.

Abstract: Quantum cryptography makes use of the quantum-mechanical behavior of nature for the design and analysis of cryptographic schemes. Optimally - yet not always - quantum cryptography allows for the design of cryptographic schemes whose security is guaranteed solely by the laws of nature. This is in sharp contrast to (most of) the cryptographic schemes that are in use, which can be broken with sufficient computing power.
In this talk, I will briefly recall the traditional application, quantum key distribution (QKD), but then I will mainly focus on recent new developments in quantum cryptography to 2-party cooperation (2PC). Whereas QKD protects honest parties against dishonest outsiders, 2PC protects against possibly dishonest insiders. While it has been known for a while that quantum 2PC schemes cannot be secure against adversaries that are limited merely by the laws of quantum mechanics, several quantum cryptographic schemes for different 2PC tasks have recently been proposed whose security relies on the hardness of reliably storing large quantum states. Although the laws of quantum mechanics do not forbid the possibility of storing large quantum states, from a technological point of view it appears to be an extremely difficult problem, and therefore it is well suited to base cryptographic security upon it.
I will discuss some example quantum 2PC schemes, e.g. an identification scheme that allows to prove knowledge of a password (or PIN) without actually giving away any information on it. While it is quite easy to build up some intuition that these schemes should be secure (assuming a limit on the adversary's quantum storage capabilities), proving them rigorously secure is highly non-trivial and requires new results in quantum information theory, which I will briefly discuss (if time permits).

Abstract: Linear optics quantum computation forms a promising and popular road towards a quantum computer, because it does not rely on interactions between the photons, using only linear optical elements such a beam splitters, mirrors, and phase shifters. The electronic analogue, a quantum computer that does not rely on electron-electron interactions, might be equally promising (not so much because the interactions between electrons are weak, but because it is difficult to control them). A "no-go theorem" suggests that the computational power of free fermions (such as electrons) does not go beyond that of a classical computer. Here we show how to work around this obstacle, and describe a CNOT gate for non-interacting electrons.

Abstract: We have recently produced the first two-dimensional array of magnetic microtraps for ultracold atoms based on patterned magnetic films. We typically prepare hundreds of tightly confined and optically resolved mesoscopic atom clouds containing 10-1000 atoms per site. The design of our atom chip allows us to shift these clouds along the array and to locally manipulate single sites using focused lasers. This system forms an ideal starting point for scalable quantum information experiments.
Careful analysis of absorption images reveals shot-to-shot fluctuations of the atom number well below the standard Poissonian "shot noise" level, with a Fano factor of 0.6, for ensembles containing between 50 and 300 atoms. This could be used to prepare ensembles with well-defined atom numbers and hence improved fidelity of operations on ensemble qubits. We are currently investigating the site-selective creation of hyperfine coherence and the use of highly excited Rydberg states to mediate long-range interactions between neighbouring microtraps. These could be used to entangle mesoscopic ensembles of atoms.